The os trigonum (plural: os trigona) is one of the ossicles of the foot and can be mistaken for a fracture.
It sits posterior to the talus on the lateral foot radiograph and represents a failure of fusion of the lateral tubercle of the posterior process. It is estimated to be present in ~7% of adults . Bilateral os trigona are seen in 2% of individuals . The ossicle usually forms between 7-13 years of age and fuses with the talus in a majority of patients by approximately age 17 , otherwise persisting as an os trigonum.
General imaging differential considerations include: